1. Field of the Invention
This invention relates to radio transmitters and, more particularly, to radio transmitters for emergency vehicles for transmitting a relatively low powered radio signal on a plurality of AM and FM frequencies along the path of emergency vehicles.
2. Description of the Prior Art
The generation of radio signals for overriding standard AM and FM broadcast frequencies by emergency vehicles is not new. However, the prior art systems utilize an oscillator sweeping back and forth across the particular frequency band and modulation is impressed upon the swept output of the oscillator. As a receiver receives the swept signal, the detected output is simply the sweep rate of the transmitter. Accordingly, any modulation impressed upon the carrier frequencies is generally below the level of the carrier. The result is that the emergency message being broadcast is generally relatively unintelligible.
Several patents illustrate the prior art concepts in emergency vehicle transmissions. For example, U.S. Pat. No. 2,994,765 (Adam) discloses an electronic system that sweeps a particular band to cause interference on all of the frequencies of the receiver. The interference either simulates a siren or else broadcasts a message.
U.S. Pat. No. 3,660,811 (Vail et al) discloses a radio system in which the frequency of a transmitter is swept acorss the receiver band to provide a warning sound. The tuning system of thr transmitter is mechanically activated to move tuning slugs. Both AM And FM receivers are affected by the directional jamming signals from the system.
U.S. Pat. No. 3,673,560 (Barsh et al) discloses a system that broadcasts to an adapter connected to a standard receiver. The receiver apparatus requires that an adapter be pre-connected to the receiver. The regular radio reception is overrridden by the adapter, so that the broadcast message from an emergency vehicle is received. It will be noted that this system requires an adapter, and accordingly vehicles whose radios do not have the adapter will not receive the message from the emergency vehicle.
U.S. Pat. No. 3,710,313 (Kimball et al) discloses a system which also requires an additional device or adapter connected to a radio receiver in a vehicle. The apparatus is specifically adapted to FM radios. Again, a special adapter is required, and vehicles without the adapter will not receive the broadcast message.
U.S. Pat. No. 3,949,300 (Sadler) discloses a transmitter which sweeps across a frequency band. Sweepingis accomplished at an audio rate. Both AM and FM frequencies are affected.
U.S. Pat. No. 4,443,790 (Bishop) discloses a system in which AM and FM transmitters in an emergncy vehicle are driven by oscillators which cover the entire AM and FM frequency bands. The sweeping of the bands is accomplished at a predetermined rate.